Table 11.5Typical Pressures in Humans
Body system Gauge pressure in mm HgBlood pressures in large arteries (resting)
Maximum (systolic) 100–140
Minimum (diastolic) 60–90
Blood pressure in large veins 4–15
Eye 12–24
Brain and spinal fluid (lying down) 5–12
Bladder
While filling 0–25
When full 100–150
Chest cavity between lungs and ribs −8 to −4
Inside lungs −2 to +3
Digestive tract
Esophagus −2
Stomach 0–20
Intestines 10–20
Middle ear <1Blood Pressure
Common arterial blood pressure measurements typically produce values of 120 mm Hg and 80 mm Hg, respectively, for systolic and diastolic
pressures. Both pressures have health implications. When systolic pressure is chronically high, the risk of stroke and heart attack is increased. If,
however, it is too low, fainting is a problem.Systolic pressureincreases dramatically during exercise to increase blood flow and returns to normal
afterward. This change produces no ill effects and, in fact, may be beneficial to the tone of the circulatory system.Diastolic pressurecan be an
indicator of fluid balance. When low, it may indicate that a person is hemorrhaging internally and needs a transfusion. Conversely, high diastolic
pressure indicates a ballooning of the blood vessels, which may be due to the transfusion of too much fluid into the circulatory system. High diastolic
pressure is also an indication that blood vessels are not dilating properly to pass blood through. This can seriously strain the heart in its attempt to
pump blood.
Blood leaves the heart at about 120 mm Hg but its pressure continues to decrease (to almost 0) as it goes from the aorta to smaller arteries to small
veins (seeFigure 11.37). The pressure differences in the circulation system are caused by blood flow through the system as well as the position of
the person. For a person standing up, the pressure in the feet will be larger than at the heart due to the weight of the blood(P=hρg). If we
assume that the distance between the heart and the feet of a person in an upright position is 1.4 m, then the increase in pressure in the feet relative
to that in the heart (for a static column of blood) is given by
ΔP= Δhρg=(1.4 m)⎛ (11.53)
⎝1050 kg/m
3 ⎞
⎠⎛⎝9.80 m/s
2 ⎞
⎠= 1.4×10
(^4) Pa = 108 mm Hg.
Increase in Pressure in the Feet of a Person
ΔP= Δhρg=(1.4 m)⎛ (11.54)
⎝1050 kg/m
3 ⎞
⎠⎛⎝9.80 m/s
2 ⎞
⎠= 1.^4 ×10
(^4) Pa = 108 mm Hg.
Standing a long time can lead to an accumulation of blood in the legs and swelling. This is the reason why soldiers who are required to stand still for
long periods of time have been known to faint. Elastic bandages around the calf can help prevent this accumulation and can also help provide
increased pressure to enable the veins to send blood back up to the heart. For similar reasons, doctors recommend tight stockings for long-haul
flights.
Blood pressure may also be measured in the major veins, the heart chambers, arteries to the brain, and the lungs. But these pressures are usually
only monitored during surgery or for patients in intensive care since the measurements are invasive. To obtain these pressure measurements,
qualified health care workers thread thin tubes, called catheters, into appropriate locations to transmit pressures to external measuring devices.
The heart consists of two pumps—the right side forcing blood through the lungs and the left causing blood to flow through the rest of the body
(Figure 11.37). Right-heart failure, for example, results in a rise in the pressure in the vena cavae and a drop in pressure in the arteries to the lungs.
Left-heart failure results in a rise in the pressure entering the left side of the heart and a drop in aortal pressure. Implications of these and other
pressures on flow in the circulatory system will be discussed in more detail inFluid Dynamics and Its Biological and Medical Applications.
Two Pumps of the Heart
The heart consists of two pumps—the right side forcing blood through the lungs and the left causing blood to flow through the rest of the body.
CHAPTER 11 | FLUID STATICS 387